Electrical circuit



p i 6, 8- R. E. MEAGHER 2,439,313

ELECTRICAL CIRCUIT Filed Jan. 24, 1945 INVEN TOR. RALPH E.

MEAGHER ATTORNEY Patented Apr. 6, 1948 ELECTRICAL CIRCUIT Application January 24, 1945, Serial :No. 574 ,3 l 1 The present invention relates generally to circuits for producing voltage variations for deflecting a cathode ray beam in a cathode ray tube, and more particularly to such circuits as employed in conjunction with electromagnetic de-. fleeting coils. a

In deflecting a cathode ray beam by means of electromagnetic deflecting coi1s,'it is diiiicult to maintain a strict linearity of deflection in view of the inductance of the electromagnetic deflecting coils and in view of certain circuit parameters associated'with the coils.

Accordingly, it is the principal object of this invention to provides, circuit arrangement for driving a linear sawtooth current through the electromagnetic deflection coils of a cathode ray tube.

It is another object of this invention to provide a circuit of the above type which will aflord a substantially linear sweep output voltage of short duration from a square wave input voltage with a minimum number of circuit elements.

Various other purposes andadvantages of the present invention will be more apparent to those skilled in the art from the following detailed description when considered in connection with the annexed drawing. The scope of the invention will be pointed out in the accompanying claims.

Referring now to the drawing, schematically shown is a deflection circuit for driving linear sawtooth current through the electromagnetic deflection coil ill of a cathode ray oscilloscope tube (not shown herein). The circuit comprises a voltage generator including a pentode tube H and a bridge arrangement incorporating a beam power tube l2. A negative square wave gate voltage such as that shown byform I3 is applied to the control grid circuit of pentode l l at terminals it with a magnitude rendering said tube nonconductive, thereby causing condenser E5 in the plate circuit thereof to commence accumulating I a charge at a rate dependent upon the capacity of the condenser and the values of variable resisters l6 and ll. Condenser l5 continues to accumulate charge for the duration of negative gate i3 and the resultant sawtooth wave and the square wave developed across the resistor it are directly impressed upon the control grid of beam power tube 12. The resistance-capacitance network (condenser lfi'and resistors l9 and I1) is arranged so that only the linear portion of the condenser charging curve is employed in the rising excursion of the sawtooth wave. The bias on pentode ll is adjusted so'that at the conclusion 4. Claims. (01, 315%2177 of the negative. gate wave l3. the, pentode His rendered conductive, thereby rapidly discharging.

with resistor 22 behaves essentially constant current generator? that is, 'it'has an extremely high resistance 'as' seen at the plate of bea'r'ntube l 2 "which means that the" current therethrou'gh is substantially independent of variations in'p'otential at vertex y. Chokecoil I9'is'desig'ned to have a far greater inductive rea'ctanc'e than the reactances of deflection'coil H]; l

The bridge is'ar'ranged so that it is essentially balancedfor D.1Clpotentia1s t11e potential of vertex'ybeing approiiiiiiatelyfequal to that of vertex 2. 'Under' these. condition'slittle or no current is flowing in coil 191 'Its' magnetic field is therefore zero, and hence the-beam of the cathode ray tube. is" undefl'ected'. 'Thebridge is however unbalanced for1A.C.potentia1s'so' that anyA.-C. signal resulting in a change of impedance in the arm consisting o'ffltube l2 and resistor 22 will pass through the deflection coiI Hi. The tube -l2 is thus used'so that it passes a quiescent current large enough to makeuse r me linear portion of the tube characteristic. This quiescent current flowing throughtube 12 in the interval between deflection waves isbalanced by a curr nt flowing through resistor 2! sothat verti'ces 'y[ and .z are substantially at the samepotential.-

In operation; as 'tliepotential at the. control grid of beam tube [2 rises' linearly durin'g'the sweep, the impedance of the bridge arm incorporating beam tube 12 accordingly decreases'in a linear manner. By' reason of the large reactanc'eof choke; lSthefcurr'ent therethrough is substantiall constant at all times? The decreasing impedance of beam tubel l2 tends to cause, more current to fiow through choker [9 than the average value which results in'Yan induced voltage-in the latter,'tliushausing'vertexlifto decrease in potential. 'Thisteffects anunbalancefin bridge circuit and, m-con'stuence, current flows through deflection coil Ill ironiflvertex' a-tofverteiiyl As the. impedance of tube 12 changes linearlywith time, the current throughldeflection coil "l U 'will also change imeanywitn time. After" each sweep,

attains quiescent conditions are rapidly restored by virtue of the direct coupling between the sawtooth generator and the grid of beam tube [2.

The initial voltage on the grid of beam tube I2 is preferably made of such magnitude whereby during the period between successive sweeps the vertex 3 is slightly higher or lower in potential than vertex 2. This produces a slight initial current in deflection coil I of an intensity suflicient to cancel the eirect of residual magnetism in the magnetic path of said coil thereby insuring that the electron beam of the cathode ray tube will commence at the zero position with the start of each sweep.

This initial current through the deflection coil It may be secured in one method by varying the normal bias on the control grid of beam tube 12. This is accomplished by the adjustment of the bias of tube ll. Another method or obtaining an initial current through deflection coil I0 is by varying either resistor or 2| to slightly unbalance the bridge arrangement. This method is plainly a more satisfactory one. Resistor 23 connected in series with deflection coil Ill is of small ohmic value and permits monitoring the current through the deflection coil.

The advantage and effect of direct coupling between the sweep generator-circuit and deflection coil It is that the quiescent current in the deflection coil H] is made substantially independent of the repetition rate and the duration of th sawtooth current wave.

Adjustment of resistor I! adjusts the slope of the sawtooth voltage wave produced by tube II.

Resistor 24 prevents the sawtooth voltage wave produced by tube H from returning to its quiescent state in too short a time.

Resistance I 6 is adjusted to add a small square wave in voltage to the sawtooth wave in order to compensate for any resistance which may be in coil it or for a damping resistor which may be placed in parallel with coil I0. It will be seen that both a sawtooth and a square wave is impressed on the grid-cathode circuit of the tube 12.

Thus there has been shown a circuit arrangement which is particularly useful for driving linear sawtooth current wave through the magnetic deflection coils of a cathode ray tube.

While there has been described what is at present considered a preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention. For example; the invention is not lim- 7 ited to the use of a beam tube l2 but may in place thereof incorporate any other tube having suitable characteristics. It is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

V 1. The combination in a circuit for driving current through the electromagnetic deflection coil of a cathode ray tube comprising a four-arm bridge arrangement having the deflection coil shunted across one pair of vertices, a source of direct current potential applied across the other pair of vertices of said bridge, a high reactance choke inserted in the first arm of said bridge, an electron discharge device having its plate circuit inserted in the second arm of said bridge adjacent to said first arm, a first resistor inserted in the third arm of said bridge, a second resistor inserted in the fourth arm of said bridge, the values of said first and second resistor being 4 such as to substantially balance the bridge when said discharge device is in a quiescent condition, means for slightly unbalancing the bridge arrangement whereby a slight current is made to flow in the deflection coil when said discharge device is in a quiescent condition in order to overcome the residual magnetism of said deflection coil, a saw-tooth voltage generator, and means for directly applying the output of said generator to the control circuit of said discharge device whereby the bridge is unbalanced and a current is made to flow through the deflection coil whose form is identical to the saw-tooth applied to said discharge device.

2. The combination in a circuit for driving current through the electromagnetic deflection coil of a cathode ray tube comprising a four-arm bridge arrangement having the deflection coil shunted across one pair of vertices, a source of direct current potential applied across the other pair of vertices of said bridge, a high reactance choke inserted in the first arm of said bridge, an electron discharge device having its plate circuit inserted in the second arm of said bridge adjacent to said first arm whereby the said reactance choke will develop an induced voltage corresponding in form to the variation in current flow through said second named arm, a first resistor inserted in the third arm of said bridge, a second resistor inserted in the fourth arm of said bridge, the values of said first and second resistor being such as to balance the bridge when said discharge device is in a quiescent condition, means for slightly unbalancing the bridge arrangement whereby a slight current is made to flow in the deflection coil when said discharge device is in a quiescent condition in order to overcome the residual magnetism of said deflection coil, a voltage generator to produce both sawtooth and square waves, and means for directly applying the output of said generator to the control circuit of said discharge device.

3. The device according to claim 2 wherein th voltage generator comprises an electron discharge device adapted to be rendered non-conductive upon application to its control circuit of a negative square wave gate voltage and a resistancecapacitance circuit connected between the anode of said device and ground, the said resistancecapacitance circuit being adapted to develop a sawtooth wave across the capacitance and a square wave across the said resistance.

4. A system to produce a linear deflection in an electromagnetic coil of a cathode ray tube comprising a source of sawtooth and square voltage waves and a bridge arrangement having the said electromagnetic coil connected across one pair of vertices and a source of direct current poten tial connected across the other pair of vertices; the said source of voltage waves consisting of a first electron discharge device adapted to be cut off upon application of a negative square wave gate voltage to its control circuit and a variable resistance-capacitance circuit connected between the anode of said first electron discharge device and ground to generate both the sawtooth wave and a square wave; the said bridge arrangement consisting of four legs successively composed of the plate circuit of a second electron discharge device having its control circuit electrically connected to the aforesaid resistance-capacitance circuit and adapted to pass a quiescent current whereby the linear portion of the tube characteristic may be utilized, a high inductance reactance choke to generate an induced voltage that RALPH E. MEAGHER.

REFERENCES CITED The following references are of record in the file of this patent:

5 UNITED STATES PATENTS Number Name Date 1,989,394 Aull Jan. 29, 1935 2,104,211 Soller Jan. 4, 1938 10 2,140,102 Bowman et a1 Dec. 13, 1938 2,313,966 Pooh Mar. 16, 1943 

